In current linear tape open (LTO) and enterprise-level tape drives, variable-frequency oscillator (VFO) patterns are a fundamental component of data recording and synchronization. Such VFO patterns may be used to align clock frequencies and bit locations. They may also be used to perform master resets, thereby causing circuits and/or timing to be reset to initial conditions. If a VFO pattern that exists on tape is not detected, or data that is not a VFO pattern is mistakenly determined to be a VFO pattern, serious systematic problems may occur when attempting to read data from tape. In some cases, data on the magnetic tape may not be recoverable
Although VFO patterns are intended to be unique from patterns encountered in normal data, matching patterns (hereinafter referred to as “false VFO patterns”) may still occur in normal recorded data. One approach to avoid this problem is to require that multiple tracks contain VFO patterns simultaneously. This approach works well in cases where data on each track is independent, since the data on each track is unlikely to contain the same false VFO pattern simultaneously. However, in cases where multiple tracks have the same or very similar data, such as in cases where certain test data patterns are written to tape, this approach may fail.
Another approach to avoid false VFO detection is to use a longer VFO pattern and require that all bits match the VFO pattern. One drawback of this approach is that it increases the probability that actual VFO patterns will be missed, since any error or media defect may cause the longer VFO patterns to go undetected. Another drawback of this approach is that longer VFO patterns reduce storage format efficiency, since the longer VFO patterns consume more storage space.
Yet another approach to avoid false VFO detection is to adjust the size of a VFO-detection window. However, adjusting the size of the VFO-detection window provides a tradeoff. If the VFO-detection window is large, the probability of detecting an actual VFO pattern is reduced, since any error or media defect may cause the actual VFO pattern to go undetected. If the VFO-detection window is small, the probability of detecting a false VFO pattern is increased.
In view of the foregoing, what are needed are improved apparatus and methods to avoid or minimize false VFO detection. Ideally, such apparatus and methods will minimize the chance that certain types of data, such as test data or other repeating data patterns, will cause a VFO to be falsely detected. Further needed are apparatus and methods to provide the above-stated benefits without reducing storage format efficiency.